Three terms that appear in any discussion of stability and control are: (1) Stability, (2) maneuverability, and (3) controllability. Stability is the characteristic of an aircraft which tends to cause it to fly (hands off) in a straight and level flight path.
Maneuverability is the ability of an aircraft to be directed along a desired flight path and to withstand the stresses imposed. Controllability is the quality of the response of an aircraft to the pilot's commands while maneuvering the aircraft.
Static Stability An aircraft is in a state of equilibrium when the sum of all the forces acting on the aircraft and all the moments is equal to zero. An aircraft in equilibrium experiences no accelerations, and the aircraft continues in a steady condition of flight. A gust of wind or a deflection of the controls disturbs the equilibrium, and the aircraft experiences acceleration due to the unbalance of moment or force.
The three types of static stability are defined by the character of movement following some disturbance from equilibrium. Positive static stability exists when the disturbed object tends to return to equilibrium. Negative static stability or static instability exists when the disturbed object tends to continue in the direction of disturbance. Neutral static stability exists when the disturbed object has neither the tendency to return nor continue in the displacement direction, but remains in equilibrium in the direction of disturbance. These three types of stability are illustrated in figure 2-11.
Dynamic Stability While static stability deals with the tendency of a displaced body to return to equilibrium, dynamic stability deals with the resulting motion with time.
If an object is disturbed from equilibrium, the time history of the resulting motion defines the dynamic stability of the object. In general, an object demonstrates positive dynamic stability if the amplitude of motion decreases with time. If the amplitude of motion increases with time, the object is said to possess dynamic instability.
Any aircraft must demonstrate the required degrees of static and dynamic stability. If an aircraft were designed with static instability and a rapid rate of dynamic instability, the aircraft would be very difficult, if not impossible, to fly. Usually, positive dynamic stability is required in an aircraft design to prevent objectionable continued oscillations of the aircraft.
Tendency to return to equilibrium
- Equilibrium A. Positive Static Stability Equilibrium Tendency to continue in displacement direction B. Negative static stabiliy or static instability Equilibrium encountered at any point of displacement C. Neutral static stability FIGURE 2-11. Static stability.
Longitudinal Stability When an aircraft has a tendency to keep a constant angle of attack with reference to the relative wind-that is, when it does not tend to put its nose down and dive or lift its nose and stall--it is said to have longitudinal stability. Longitudinal stability refers to motion in pitch. The horizontal stabilizer is the primary surface which controls longitudinal stability. The action of the stabilizer depends upon the speed and angle of attack of the aircraft.
Figure 2-12 illustrates the contribution of tail hift to stability. If the aircraft changes its angle of attack, a change in lift takes place at the aerody.
namic center (center of pressure) of the horizontal stabilizer.
Increase in lift Aerodynamic center of horizontal tail FIGURE 2-12. Producing tail lift.
Under certain conditions of speed, load, and angle of attack, the flow of air over the horizontal stabilizer creates a force which pushes the tail up or down. When conditions are such that the airflow creates equal forces up and down, the forces are said to be in equilibrium. This condition is usually found in level flight in calm air.
Directional Stability Stability about the vertical axis is referred to as directional stability. The aircraft should be de.
signed so that when it is in straight and level flight it remains on its course heading even though the pilot takes his hands and feet off the controls. If an aircraft recovers automatically from a skid, it has been well designed and possesses good directional balance. The vertical stabilizer is the primary surface which controls directional stability.
As shown in figure 2-13, when an aircraft is in a sideslip or yawing, the vertical tail experiences a change in angle of attack with a resulting change in lift [not to be confused with the lift created by the wing]. The change in lift, or side force, on the vertical tail creates a yawing moment about the center of gravity which tends to return the aircraft to its original fight path.
Relative wind Tail moment / arm Center of gravity Change in tail lift FIGURE 2-13. Contribution of vertical tail to directional stability.
Sweptback wings aid in directional stability. If the aircraft yaws from its direction of flight, the wing which is farther ahead offers more drag than the wing which is aft. The effect of this drag is to hold back the wing which is farther ahead, and to let the other wing catch up.
Directional stability is also aided by using a large dorsal fin and a long fuselage.
The high Mach numbers of supersonic flight reduce the contribution of the vertical tail to directional stability. To produce the required directional stability at high Mach numbers, a very large vertical tail area may be necessary. Ventral (belly) fins may be added as an additional contribution to directional stability.
Lateral Stability We have seen that pitching is motion about the aircraft's lateral axis and yawing is motion about its vertical axis. Motion about its longitudinal (fore and aft) axis is a lateral or rolling motion. The tendency to return to the original attitude from such motion is called lateral stability.
The lateral stability of an airplane involves consideration of rolling moments due to sideslip. A sideslip tends to produce both a rolling and a yawing motion. If an airplane has a favorable rolling moment, a sideslip will tend to return the airplane to a level flight attitude.
The principal surface contributing to the lateral stability of an airplane is the wing. The effect of the geometric dihedral (figure 2-14) of a wing is a powerful contribution to lateral stability. As shown in figure 2-14, a wing with dihedral develops stable rolling moments with sideslip. With the relative wind from the side, the wing into the wind is subject to an increase in angle of attack and develops an increase in lift. The wing away from the wind is subject to a decrease in angle of attack and develops less lift. The changes in lift effect a rolling moment tending to raise the windward wing.
When a wing is swept back, the effective dihedral increases rapidly with a change in the lift coefficient of the wing. Sweepback is the angle between a line perpendicular to the fuslage center line and the quarter chord of each wing airfoil section.
Sweepback in combination with dihedral causes the dihedral effect to be excessive. As shown in figure 2-15, the swept-wing aircraft in a sideslip has the wing that is into the wind operating with an effective decrease in sweepback, while the wing out of the wind is operating with an effective increase in 38
Plane of symmetry Line perpendicular to Dihedral angle Effective increase in lift plane of symmetry due to sideslip Relative wind Decrease in lift due to sideslip FicURE 2-14. Contribution of dihedral to lateral stability.
sweepback. The wing into the wind develops more lift, and the wing out of the wind develops less.
This tends to restore the aircraft to a level flight attitude.
The amount of effective dihedral necessary to produce satisfactory flying qualities varies greatly with the type and purpose of the aircraft. Generally, the effective dihedral is kept low, since high roll due to sideslip can create problems. Excessive dihedral effect can lead to Dutch Roll, difficult rudder coordination in rolling maneuvers, or place extreme demands for lateral control power during crosswind takeoff and landing.
CONTROL Control is the action taken to make the aircraft follow any desired flight path. When an aircraft is said to be controllable, it means that the craft responds easily and promptly to movement of the controls. Different control surfaces are used to control the aircraft about each of the three axes.
Moving the control surfaces on an aircraft changes the airfiow over the aircraft's surface. This, in turn, creates changes in the balance of forces acting to keep the aircraft flying straight and level.
Reduced sweep of wing into wind results in increased lift Increased sweep of wing out of wind results in decreased lift Relative wind Relative wind FIGURE 2-15. Effect of sweepback on lateral stability.
Rudder Secondary Group Included in the secondary group are the trim tabs and spring tabs. Trim tabs (figure 2-17) are small airfoils recessed into the trailing edges of the primary control surfaces. The purpose of trim tabs is to enable the pilot to trim out any unbalanced condition which may exist during flight, without exert- Elevator ing any pressure on the primary controls. Lach trim tab is hinged to its parent primary control surface, but is operated by an independent control.
Spring tabs are similar in appearance to trim Spoiler tabs, but serve an entirely different purpose. Spring aileron tabs are used to aid the pilot in moving the primary control surfaces.
FICURE 2-16. Primary flight controls.
FLIGHT CONTROL SURFACES The flight control surfaces are hinged or movable airfoils designed to change the attitude of the aircraft during flight. These surfaces may be divided into three groups, usually referred to as the primary group, secondary group, and auxiliary group.
Primary Group The primary group includes the ailerons, elevators, and rudder (figure 2-16). These surfaces are used for moving the aircraft about its three axes.
Trim tab Trim tabs FIGURE 2-17. Trim tabs.
The ailerons and elevators are generally operated from the cockpit by a control stick on single-engine aircraft and by a wheel and yoke assembly on multiengine aircraft. The rudder is operated by foot pedals on all types of aircraft.
Auxiliary Group Included in the auxiliary group of flight control surfaces are the wing flaps, spoilers, speed brakes, slats, leading edge flaps and slots.
The auxiliary groups may be divided into two sub-groups. Those whose primary purpose is lift augmenting and those whose primary purpose is lift decreasing. In the first group are the flaps, both trailing edge and leading edge (slats), and slots. The lift decreasing devices are speed brakes and spoilers.
The trailing edge airfoils (Haps) increase the wing area thereby increasing lift on takeof and decrease the speed during landing. These airfoils are retractable and fair into the wing contour.
Others are simply a portion of the lower skin which extends into the airstream thereby slowing the aircraft.
Leading edge flaps are airfoils extended from and retracted into the leading edge of the wing. Some installations create a slot (an opening between the extended airfoil and the leading edge). The flap (termed slat by some manufacturers) and slot create additional lift at the slower speeds of takeoff and landing. Other installations have permanent slots built in the leading edge of the wing. At cruising speeds, the trailing edge and leading edge flaps (slats) are retracted into the wing proper.
Lift decreasing devices are the speed brakes (spoilers). In some installations, there are two types of spoilers. The ground spoiler is extended only after the aircraft is on the ground thereby assisting in the braking action. The flight spoiler 40
assists in lateral control by being extended whenever the aileron on that wing is rotated up. When actuated as speed brakes, the spoiler panels on both wings raise up--the panel on the "up" aileron wing raising more than the panel on the down aileron side. This provides speed brake operation and later control simultaneously.
Slats are movable control surfaces attached to the leading edges of the wings. When the slat is closed, it forms the leading edge of the wing. When in the open position (extended forward), a slot is created between the slat and the wing leading edge. At low airspeeds this increases lift and improves handling characteristics, allowing the aircraft to be controlled at airspeeds below the otherwise normal landing speed.
CONTROL AROUND THE LONGITUDINAL AXIS The motion of the aircraft about the longitudinal axis is called rolling or banking. The ailerons (figure 2-18) are used to control this movement.
The ailerons form a part of the wing and are located in the trailing edge of the wing toward the tips. Ailerons are the movable surfaces of an otherwise fixed-surface wing. The aileron is in neutral position when it is streamlined with the trailing edge of the wing.
Ailerons respond to side pressure applied to the control stick. Pressure applied to move the stick toward the right raises the right aileron and lowers Ailerons Vertical axis Level flight FIGURE 2-18. Aileron action.
the left aileron, causing the aircraft to bank to the right. Ailerons are linked together by control cables so that when one aileron is down, the opposite aileron is up. The function of the lowered aile ron is to increase the lift by increasing the wing camber. At the same time, the down aileron also creates some additional drag since it is in the area of high pressure below the wing. The up aileron, on the opposite end of the wing, decreases lift on that end of the wing. The increased lift on the wing whose aileron is down, raises this wing. This causes the aircraft to roll about its longitudinal axis as shown in figure 2-19.
As a result of the increased lift on the wing with the lowered aileron, drag is also increased. This drag attempts to pull the nose in the direction of the high wing. Since the ailerons are used with the rudder when making turns, the increased drag tries to turn the aircraft in the direction opposite to that desired. To avoid this undesirable effect, aircraft are often designed with differential travel of the ailerons.
Differential aileron travel (figure 2-20) provides more aileron up travel than down travel for a given movement of the control stick or wheel in the cockpit.
The spoilers, or speed brakes as they are also called, are plates fitted to the upper surface of the wing. They are usually deflected upward by hydraulic actuators in response to control wheel movement in the cockpit. The purpose of the spoilers is to disturb the smooth airflow across the top of the airfoil thereby creating an increased amount of drag and a decreased amount of lift on that airfoil.
Spoilers are used primarily for lateral control, When banking the airplane, the spoilers function with the ailerons. The spoilers on the up aileron side raise with that aileron to further decrease the lift on that wing. The spoiler on the opposite side remains in the faired position. When the spoilers are used as a speed brake, they are all deflected upward simultaneously. A separate control lever is provided for operating the spoilers as speed brakes.
While we tend to think of a spoiler as being a fairly complicated, controlled device, we should keep in mind that some are not controllable. Some spoilers are automatic in operation in that they come into effect only at a high angle of attack. This 41
Longitudinal axis Roll direction
FIGURE 2-21. Fixed spoilers or stall strip.
arrangement keeps them out of the slipstream at cruise and high speeds.
• A fixed spoiler may be a small wedge affixed to the leading edge of the airfoil as shown in figure
2-21. This type spoiler causes the inboard portion of the wing to stall ahead of the outboard portion which results in aileron control right up to the occurrence of complete wing stall.
Direction of rudder movement Direction of airplane motion Direction of pedal movement.
Relative wind - FIGURE 2-22. Rudder action.
Use extreme accuracy in positioning leading edge spoilers when re-installing them after they have been removed for maintenance, Improper positioning may result in adverse stall characteristics. Always follow the manufacturers' instructions regarding location and method of attachment.
CONTROL AROUND THE VERTICAL AXIS Turning the nose of the aircraft causes the aircraft to rotate about its vertical axis. Rotation of the aircraft about the vertical axis is called yawing.
This motion is controlled by using the rudder as illustrated in figure 2-22.
The rudder is a movable control surface attached to the trailing edge of the vertical stabilizer. To turn the aircraft to the right, the rudder is moved to the right. The rudder protrudes into the airstream, causing a force to act upon it. This is the force necessary to give a turning movement about the center of gravity which turns the aircraft to the right. If the rudder is moved to the left, it induces a
- Bellcrank Pivot • - Pivot counterclockwise rotation and the aircraft similarly turns to the left. The rudder can also be used in controlling a bank or turn in fight.
The main function of the rudder is to turn the aircraft in flight, This turn is maintained by the side pressure of the air moving past the vertical surfaces. When an aircraft begins to slip or skid, rudder pressure is applied to keep the aircraft headed in the desired direction (balanced).
Slip or sideslipping refers to any motion of the aircraft to the side and downward toward the inside of a turn. Skid or skidding refers to any movement upward and outward away from the center of a turn.
CONTROL AROUND THE LATERAL AXIS When the nose of an aircraft is raised or lowered, it is rotated about its lateral axis. Elevators are the movable control surfaces that cause this rotation (figure 2-23). They are normally hinged to the trailing edge of the horizontal stabilizer.
Up
- — 0° Neutral Down FICURE 2-23. Elevator action.
The elevators are used to make the aircraft climb or dive and also to obtain sufficient lift from the wings to keep the aircraft in level flight at various speeds.
The elevators can be moved either up or down. If the elevator is rotated up, it decreases the lift force on the tail causing the tail to lower and the nose to rise. If the elevator is rotated downward, it increases the lift force on the tail causing it to rise and the nose to lower. Lowering the aircraft's nose increases forward speed, and raising the nose decreases forward speed.
Some aircraft use a movable horizontal surface called a stabilator (figure 2-24). The stabilator serves the same purpose as the horizontal stabilizer and elevator combined. When the cockpit control is moved, the complete stabilator is moved to raise or lower the leading edge, thus changing the angle of attack and the amount of lift on the tail surfaces.
Pivot FIcURE 2-24. Movable horizontal stabilator.
Aircraft empennages have been designed which combine the vertical and horizontal stabilizers. Such empennages have the stabilizers set at an angle as shown in figure 2-25. This arrangement is referred to as a butterfly or vee tail.
The control surfaces are hinged to the stabilizers at the trailing edges. The stabilizing portion of this arrangement is called a stabilator, and the control portion is called the ruddervator. The ruddervators can be operated both up or both down at the same Stabilizers time. When used in this manner, the result is the same as with any other type of elevator. This action is controlled by the stick or control column.
The ruddervators can be made to move opposite each other by pushing the left or right rudder pedal (figure 2-26). If the right rudder padel is pushed, the right ruddervator moves down and the left ruddervator moves up. This produces turning moments to move the nose of the aircraft to the right.
Ruddervator FIGURE 2-25. A butterfly or vee tail.
Direction of yaw Direction of surface travel
- Lateral axis Direction of pitch (A) (B) FIGURE 2-26. Ruddervator action. (A) functioning as an elevator; (B) functioning as a rudder.
TABS Even though an aircraft has inherent stability, it does not always tend to fly straight and level. The weight of the load and its distribution affect stability. Various speeds also affect its flight characteristics. If the fuel in one wing tank is used before that in the other wing tank, the aircraft tends to roll toward the full tank. All of these variations require constant exertion of pressure on the controls for correction. While climbing or gliding, it is necessary to apply pressure on the controls to keep the aircraft in the desired attitude.
To offset the forces that tend to unbalance an aircraft in flight, ailerons, elevators, and rudders are provided with auxiliary controls known as tabs.
These are small, hinged control surfaces (figure 2-27) attached to the trailing edge of the primary control surfaces. Tabs can be moved up or down by means of a crank or moved electrically from the cockpit. These tabs can be used to balance the 45
forces on the controls so that the aircraft flies straight and level, or may be set so that the aircraft maintains either a climbing or gliding attitude.
Control horn Control surface Fixed surface Tab A - TRIM TAB Variable linkage Hom free to
- pivot on hinge axis
B - SERVO TAB
- Control horn
C - BALANCE TAB — Control horn D - SPRING TAB Spring cartridge FIGURE 2-27. Flight control trim tab types.
Trim Tabs Trim tabs trim the aircraft in flight. To trim means to correct any tendency of the aircraft to move toward an undesirable flight attitude. Trim tabs control the balance of an aircraft so that it maintains straight and level flight without pressure on the control column, control wheel, or rudder pedals. Figure 2-27A illustrates a trim tab.
Note that the tab has a variable linkage which is adjustable from the cockpit. Movement of the tab in one direction causes a deflection of the control surface in the opposite direction. Most of the trim tabs installed on aircraft are mechanically operated from the cockpit through an individual cable system.
However, some aircraft have trim tabs that are operated by an electrical actuator. Trim tabs are either controlled from the cockpit or adjusted on the ground before taking off. Trim tabs are installed on elevators, rudders, and ailerons.
Servo Tabs Servo tabs (figure 2-27B) are very similar in operation and appearance to the trim tabs just diacussed. Servo tabs, sometimes referred to as flight tabs, are used primarily on the large main control surfaces. They aid in moving the control surface and holding it in the desired position. Only the servo tab moves in response to movement of the cockpit control. (The servo tab horn is free to pivot to the main control surface hinge axis.) The force of the airflow on the servo tab then moves the primary control surface. With the use of a servo tab less force is needed to move the main control surface.
Balance Tabs A balance tab is shown in figure 2-27C. The linkage is designed in such a way that when the main control surface is moved, the tab moves in the opposite direction. Thus, aerodynamic forces, acting on the tab, assist in moving the main control surface.
Spring Tabs Spring tabs (figure 2-27D) are similar in appearance to trim tabs, but serve an entirely different purpose. Spring tabs are used for the same purpose as hydraulic actuators; that is, to aid in moving a primary control surface. There are various spring arrangements used in the linkage of the spring tab.
On some aircraft, a spring tab is hinged to the trailing edge of each aileron and is actuated by a spring-loaded push-pull rod assembly which is also linked to the aileron control linkage. The linkage is connected in such a way that movement of the aileron in one direction causes the spring tab to be deflected in the opposite direction. This provides a balanced condition, thus reducing the amount of force required to move the ailerons.
The deflection of the spring tabs is directly proportional to the aerodynamic load imposed upon the aileron; therefore, at low speeds the spring tab remains in a neutral position and the aileron is a direct manually controlled surface. At high speeds, however, where the aerodynamic load is great, the tab functions as an aid in moving the primary control surface.
To lessen the force required to operate the control surfaces they are usually balanced statically and aerodynamically. Aerodynamic balance is usually achieved by extending a portion of the con.
Hinge line Overhang Handley-Page Friese FIGURE 2-28. Three forms of aerodynamic balance.
trol surface ahead of the hinge line. This utilizes the airflow about the aircraft to aid in moving the surface. The various methods of achieving aerodynamic balance are shown in figure 2-28.
Static balance is accomplished by adding weight to the section forward of the hinge line until it weighs the same as the section aft of it. When repairing a control surface use care to prevent upsetting or disturbing the static balance. An unbalanced surface has a tendency to flutter as air passes over it.
High-Lift Devices High-lift devices are used in combination with airfoils in order to reduce the takeoff or landing speed by changing the lift characteristics of an airfoil during the landing or takeoff phases. When these devices are no longer needed they are returned to a position within the wing to regain the normal characteristics of the airfoil.
Two high-lift devices commonly used on aircraft are shown in figure 2-29. One of these is known as a slot, and is used as a passageway through the leading edge of the wing. At high angles of attack the air flows through the slot and smooths out the airflow over the top surface of the wing. This enables the wing to pass beyond its normal stalling point without stalling. Greater lift is obtained with the wing operating at the higher angle of attack.
The other high-lift device is known as a flap. It is a hinged surface on the trailing edge of the wing. The flap is controlled from the cockpit, and when not in use fits smoothly into the lower surface of each wing. The use of flaps increases the camber of a wing and therefore the lift of the wing, making it possible for the speed of the aircraft to be decreased without stalling. This also permits a steeper gliding angle to be obtained as in the landing approach. Flaps are primarily used during takeoff and landing.
The types of flaps in use on aircraft include: (1) Plain, (2) split, (3) Fowler, and (4) slotted. The plain (figure 2-30) is simply, hinged to the wing and forms a part of the wing surface when raised.
The split flap (figure 2-30) gets its name from the hinge at the bottom part of the wing near the trailing edge permitting it to be lowered from the 47
PLAIN Slot Flap SPLIT ficient flaps Slots & Slots FLAPS ALONE alone G 2 @ alone, Plain wing Angle of attack 13°15° 20° 23° FOWLER FIGURE 2-29. High-lift devices.
fixed top surface. The Fowler flap (figure 2-30) fits into the lower part of the wing so that it is flush with the surface. When the flap is operated, it slides AIR backward on tracks and tilts downward at the same time. This increases wing camber, as do the other SLOTTED types of flaps. However, Fowler flaps also increase the wing area; thus, they provide added lift without unduly increasing drag. FICURE 2-30. Types of wing flaps.
The slotted flap (figure 2-30). is like the Fowler flap in operation, but in appearance it is similar to Laminar flow area the plain flap. This flap is equipped with either - Turbulent flow area tracks and rollers or hinges of a special design.
During operation, the flap moves downward and rearward away from the position of the wing. The "slot" thus opened allows a flow of air over the the airflow and to improve the efficiency of the flap. upper surface of the flap. The effect is to streamline Local free-stream velocity.
BOUNDARY LAYER CONTROL DEVICES The layer of air over the surface which is slower moving in relation to the rest of the slipstream is called the boundary layer. The initial airflow on a smooth surface (figure 2-31) gives evidence of a very thin boundary layer with the flow occurring in smooth laminations of air sliding smoothly over one another. Therefore, the term for this type of flow is the laminar boundary layer.
- Wing surface • Boundary layer thickness- FIcURE 2-31. Boundary layer characteristics.
As the flow continues back from the leading edge, friction forces in the boundary layer continue to 48